1. Field of the Invention
The present invention relates in general to a radiographic source and an associated method of making a radiographic source. The radiographic source and the associated method of making thereof are described herein in connection with the manufacture of a cobalt-60 radiographic source. The present invention also relates to a new technique for the accurate measurement of focal spot size for in particular cobalt-60 radiographic sources.
2. Background Discussion
Radiographic sources are presently known and are constructed with the use of such radioactive materials as cobalt-60 and iridium-192. A cobalt-60 radiographic source presently manufactured by the assignee herein is constructed of inner and outer stainless steel capsules with the inner capsule containing a plurality of cobalt-60 pellets; the number of cobalt-60 pellets being the function of the source size. The inner capsule is seal welded by a plug or cover also of stainless steel.
In the past attempts have been made to compact the cobalt-60 pellets. However, the compaction has not been totally effective. The techniques used to date have caused deformation of the capsule to the extent that the inner capsule could not be inserted into the outer capsule.
Accordingly, one object of the present invention is to provide an improved method of making a radiographic source in which the source radioactive material is effectively compacted so as to provide an actual effective density as high as 90% of the density of the cobalt metal. It is to be noted that this improved radiographic material compression provides a considerably higher activity in a smaller focal spot size, resulting in substantial money savings in radiography exposure time alone.
With the presently employed stainless steel inner capsule there has always been some inherent factors that yielded uncertainty over the exact focal spot size provided. There was a statistical error relating to the cobalt pellet orientation at the edges of the focal spot (edge effects). There is also distortion of the focal spot due to ineffective compression. There was uncertainty over void volume due to the ineffective compression. There was also a statistical error from the cobalt-60 specific activity which is measured before encapsulation. While the contribution from each of these factors may be only a few percent, they are additive when calculating the overall uncertainty.
The smallest focal spot size has been in demand by the industry in general for many years. However, actual verification of focal spot size has been virtually impossible due to the fact that cobalt and steel have very similar atomic numbers (cobalt=27, steel=26) and a very similar density (cobalt=8.9 gm/cc, steel=7.9 gm/cc ). This means that it is extremely difficult to separate these materials by x-ray analysis.
Accordingly, it is another object of the present invention to provide an improved radiography source and associated method of manufacture with the inner capsule construction of a material that enables adequate film contrast by x-ray radiography between the inner capsule and the radioactive material.
A further object of the present invention is to provide an improved technique for in particular accurately measuring the focal spot size for a radiographic source and for in particular a cobalt-60 radiographic source.
Still another object of the present invention is to provide an improved apparatus for compacting radioactive pellets and for particularly compacting cobalt-60 pellets in a capsule.
Still a further object of the present invention is to provide the improved apparatus as recited in the preceding object and in which the capsule is constructed of a material that has a sufficiently different atomic number in comparison to the radioactive material so that adequate film contrast by x-ray radiography can be achieve while at the same time constructing the capsule of a material with sufficient tensile strength so as to assist in preventing deformation thereof during the compacting step.